All-wheel vehicle drive-trains are increasingly made with frictional shifting elements by means of which, in each case, drive torque that is produced by an internal combustion engine, which is converted in the area of a transmission connected downstream from the internal combustion engine as a function of an overall gear ratio currently engaged in the transmission in an operating-condition-dependent manner, can be distributed in varying degrees of distribution between two driven vehicle axles of the all-wheel drive-train, to influence the driving behavior of the all-wheel vehicle, for example to increase the driving safety.
To do this the transmission capability of such shifting elements has to be changed with high control dynamics in an operating-condition-dependent manner. For that purpose electro-mechanical drive machines are often provided, which comprise an electric drive machine and a drive converter unit, with the drive converter unit being arranged between the drive machine and the shifting element. In the area of the drive converter unit, rotary drive motion of the drive machine is converted into translational movement to actuate the shifting element.
Depending on the application concerned, a transmission ratio stage is provided in the area of the drive converter unit to transform the rotary drive of the drive machine or the drive torque provided by the drive machine to a required level that depends on the application. For this, spur gear stages, worm gear transmissions, cam plates, spindle systems or ball-ramp systems are used as the transmission ratio stage.
By virtue of application-dependent design of the drive converter unit and the resulting transmission ratio between the drive machine and a pressure disk of the shifting element to be actuated by the drive machine, the necessary level of the actuating force to be applied to the shifting element in its closing direction for the desired function of the shifting element, and in turn the drive torque of the drive machine corresponding thereto and a resultant control dynamics, can be varied or adjusted.
The power of the drive machine is designed as a function of the desired control time or closing time of the shifting element, an air gap in the area of a frictional shifting element, the component elasticities of the frictional shifting element, the component elasticities of further structural elements present in the force flow, the inertia of the drive machine and the size of the transmission ratio between the drive machine and the shifting element to be actuated by the drive machine. In addition, for the design of the drive machine and the drive converter unit combined therewith, wear that takes place over the operating life of a shifting element is also taken into account since an enlargement of the air gap caused by wear increases the control path.
If the transmission capability of a frictional shifting element has to be set, as a function of a corresponding requirement during the operation of an all-wheel drive-train, at a torque value determined, for example specified, by a vehicle computer and then maintained at that level for longer operating times, then on the part of the drive machine a holding force has to be made permanently available. When the drive machine is made as an electric motor, then to maintain the level of the torque capability of the frictional shifting element the electric motor has to be permanently energized with a current sufficient to do this. However, such a permanent current flow is undesirable since on the one hand it imposes a load on the electrical and electronic components, and on the other hand it increases the fuel consumption of the vehicle.
Various measures are provided in known systems to reduce the current demand for the operation of an electric motor, during a transmission capability maintaining phase of a frictional shifting element as described above.
For example, in the transmission path between the electric motor and the frictional shifting element respective gears are provided, which are of self-locking design. Thanks to the self-locking of the additional gear system the transmission capability of the frictional shifting element can be maintained at a desired level with small electric motor actuation currents, and the transmission capability of the frictional shifting element is only changed by higher drive torques of the electric machine.
Disadvantageously, due to the low tooth efficiency in the area of a gear system designed to be self-locking, actuating systems with self-locking require a higher torque from the drive machine over the full operating range of the shifting element than do the actuating systems without self-locking. However, higher actuating forces can only be provided with drive machines of corresponding power. Electric motors with higher power, take up more fitting space and are characterized by higher power uptake.
To be able to produce an actuation system with self-locking that occupies less fitting space, a transmission ratio between the drive machine and the shifting element can be correspondingly increased, but this compromises the control dynamics to a considerable extent. Furthermore, a usually perpendicular arrangement of a worm gear or screw gearset is a space-saving design.
Alternatively, it is known to provide, in the area of the electric motor or at some other point in the force path of the control system between the electric machine and the frictional clutch, an electromagnetic brake which, in operating phases of the frictional shifting element during which the transmission capability has to be kept substantially constant, in its closed operating condition prevents a change of the transmission capability of the shifting element. In this way, when the electromagnetic brake is engaged, the actuating current of the electric motor can sometimes be considerably reduced.
However, the use of electromagnetic holding brakes entails additional control and regulation complexity and requires corresponding hardware for actuating the electromagnetic brakes, thus increasing the manufacturing and development costs. In addition, when there are frequent demands for changing the operating condition of the frictional shifting element to be actuated, then due to the system-inherent control dynamics of the electromagnetic holding brakes, the holding effect that they provide cannot be used to the desired extent, so the current needed for operating the electric motor cannot be reduced as much as desired.
Associated with electro-mechanically actuated starting clutches of transmission devices are so-termed compensation mechanisms, which assist a control process of frictional clutches and with which the current demand of an electric motor can be reduced inexpensively with little control and regulation effort. The compensation mechanisms usually comprise a spring system that is prestressed during assembly, which during the actuation of a shifting element acts in the closing direction of the shifting element and gives up its stored spring energy in a path-dependent manner. The support provided by a compensation mechanism acting in the closing direction can be specified as desired by design means and can also be configured as a function of a characteristic force curve that acts in the opening direction in the area of the shifting element to be actuated. For example the compensation mechanism can be designed as a function of the force characteristic of the shifting element such that only a small actuating force has to be provided by the electric motor and the drive converter unit in order to change the transmission capability of the shifting element, whereby the force to be applied by the electric motor only has to bring about an operating condition change in the area of the drive converter unit without additional external force.
Control systems for shifting elements that are made with compensation mechanisms or compensation devices have the advantage, compared with systems having self-locking or with an electromagnetic brake, that already during assembly a force is stored, which opposes the actuating force of the shifting element that acts in the opening direction of the shifting element as a function of the axial actuation path of the shifting element and, depending on the design of the actuating system for the shifting element, the electric motor essentially only still has to overcome a small residual force during the actuation of the shifting element. If the compensation force provided by the compensation mechanism is smaller than the opposing force that occurs during actuation in the area of the shifting element and acts in the opening direction of the shifting element, then when the electric motor is not energized the shifting element changes as required to its open condition. During operation, spring energies stored respectively in the area of the compensation device or in the area of the shifting element, which result from component elasticities of the compensation device and of the shifting element, are exchanged between the compensation device and the shifting element.
The compensation forces that can be provided by the known compensation devices are limited in embodiments of appropriately space-saving design, so that drive power produced by the electric motor during holding phases cannot be reduced to the desired extent.